The present invention relates to the field of surface science and, more particularly, to the field of surface science where there is a desire to improve a hydrophilic behavior of a surface.
Fogging frequently occurs when a cold surface suddenly comes in contact with warm, moist air. Fogging scatters light and often renders the surface translucent. Fogging severity can ultimately compromise the usefulness of the transparent material and can be a dangerous condition, for example when the fogged material is a vehicle windscreen or goggle lens.
As an example, fogging of spectacle lenses made of glass or plastic occurs when water vapor in the air condenses because of the difference between the temperatures of the air and the spectacle lenses, with the result that tiny water droplets adhere to the surfaces of the spectacle lenses and irregularly reflect or refract light in a complex manner. To prevent the occurrence of such fogging, it is known to coat the surfaces of the spectacle lenses with an anti-fogging liquid such as a hydrophilic activator so that the surface tension is decreased and the generation of water droplets is thereby avoided. However, this technique suffers from a problem in that the coated anti-fogging liquid cannot remain effective for a long period of time because it evaporates or is wiped off.
Current commodity anti-fog coatings often lose effectiveness after repeated cleanings over time, and therefore require constant reapplication to ensure their effectiveness. Numerous compositions have been proposed for anti-fog applications. For example, in U.S. Pat. No. 6,455,142 entitled, “Anti-fog coating and coated film,” Heberger et al. teach a coated polymer film that is applied to a surface and that provides an essentially streak free coated surface. Another example is U.S. Pat. No. 5,853,896 entitled, “Water repellant agent for glass,” in which Kondo et al. teach a water repellant composition for application to glass (e.g., glass windshields) where such a composition is a mixture of an organosilane and a diorganopolysiloxane. Yet another example is U.S. Pat. No. 5,804,612 entitled, “Transparent anti-fog coating,” in which Song et al. teach a transparent coating that includes a polymer, an aluminum containing crosslinker, and a surface active agent containing hydroxyl or siloxane groups.
In certain applications, such as dental mirrors, where accurate reflection of a patient's dental details is of paramount importance, and where non-toxicity is imperative, such compositions are not completely satisfactory. Nor due they provide a total solution to the anti-fogging problem. Some require lengthy curing. Some are toxic. Many will immediately dissolve and wash off from a dental mirror in the water generated by high speed drills in current use. And none provide adequate visibility for viewing details in the operative field: the proximity of a dental mirror to the tooth diffuses the reflection and obscures tooth details necessary for precise dental work.
More generally, surfaces that strongly attract (super-hydrophilic) or repel (super-hydrophobic) water are key to the two basic routes to self-cleaning, through film or droplet flow, respectively (see, R. Blossey, Nature Materials 2 (2003) 301). While theories established in the early 20th century (see, A. B. D. Cassie et al., Transactions of the Faraday Society 40 (1944) 0546; and R. N. Wenzel, Industrial and Engineering Chemistry 28 (1936) 988) were able to relate the wetting phenomena to general surface properties, the role of surface structures has been revisited recently after studies of a number of biological systems, e.g., lotus leaf (Nelumbo Nucifera), Colocasia esculenta, and Namib desert beetle (see, W. Barthlott et al., Planta 202 (1997) 1), revealed the significance of complex hierarchical microstructures to realizing extreme wetting surfaces. These studies prompted a new strategy for self-cleaning technologies based on mimicking the morphology of biological surfaces (see, L. Zhai et al., Nano Letters 4 (2004) 1349), although the primarily goal has been to achieve super-hydrophobicity.
In the meantime, TiO2 has received much recent attention as a unique photocatalyst with exciting potential for many energy and environmental applications crossing traditional disciplinary boundaries (see, e.g., X. Chen, S. S. Mao, Chemical Reviews 107 (2007) 2891). Since the discovery of UV light induced photocatalytic activity that can enhance its surface wettability (see, R. Wang et al., Nature 388 (1997) 431; and R. Wang et al., Advanced Materials 10 (1998) 135), TiO2 has been extensively used in self-cleaning and related anti-pollution, anti-bacteria applications (see, I. P. Parkin et al., Journal of Materials Chemistry 15 (2005) 1689). However, a critical challenge of TiO2-based techniques originates from the difficulty of sustaining the wetting behavior, though many drastic steps have been taken to overcome this challenge using, for instance, non-transparent (see, C. Pan et al., Materials Research Bulletin 42 (2007) 1395; and Z. Z. Gu et al., Applied Physics Letters 85 (2004) 5067) or composite systems (see, D. Lee et al., Nano Letters 6 (2006) 2305). Recent attempts of a multilayer assembly of TiO2 nanoparticles and polyethylene glycol (see, W. Y. Gan et al., Journal of Materials Chemistry 17 (2007) 952; and S. Song et al., Materials Letters 62 (2008) 3503) showed a short-lived super-hydrophilic surface without the use of UV irradiation, for which the extreme wetting behavior collapsed after exposure to a moderate temperature. From the point of view of applications, achieving super-hydrophilic surfaces from TiO2 while circumventing the need of external stimuli is regarded as the ultimate self-cleaning technology, in particular if the extreme wetting property could be combined with additional functionalities (see, C. W. Guo et al., Chemphyschem 5 (2004) 750) such as optical transparency. In fact, incorporating TiO2 on traditional transparent substrates (e.g. glass) for low reflectance, high transmittance applications has often been challenging.